A series of co[poly(ethylene terephthalate-p-oxybenzoate)] thermotropic copolyesters were prepared by the copolymerization of poly(ethylene terephthalate) (PET) as moiety (II) and p-acetoxybenzoic acid (POB) as moiety (I). The polymeric products obtained were subjected to solid-state polymerization. Characterization of the copolyesters by thermomechanical analysis shows that properties such as the coefficient of thermal expansion, transition temperature, and maximum softening rate temperature varied directly as a function of composition as well as did solid-state polymerization time. All thermomechanical data were found to increase with the solid-state polymerization time due to the increase in the degree of polymerization and the effect of annealing. The coefficient of thermal expansion behaves in a manner that is interpretable by the crystalline state of the copolyester. The relationship between the free-volume fraction and thermodynamic properties is further correlated for a more comprehensive discussion on its molecular arrangements.